Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with atransitional graphical user interface showing a first image;

FIG. 2 is a front view showing a second image;

FIG. 3 is a front view showing a third image; and,

FIG. 4 is a front view showing a fourth image.

The appearance of the transition is sequential from FIG. 1 to FIG. 4 .The process or period in which one image transitions to another forms nopart of the claimed design.

The dot-dash broken lines illustrate the display screen and form no partof the claimed design.

The other broken lines illustrate portions of the graphical userinterface and form no part of the claimed design.

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 